The single stranded DNA anti-single stranded DNA immune complex system has been shown to have a major role in the pathogenesis of the glomerulonephritis of Systemic Lupus Erythematosus. Anti-single stranded DNA antibodies have been found to be concentrated in glomerular eluates in kidneys in which single stranded DNA antigen could be localized by immunofluorescence. Anti-ribonucleoprotein antibodies were also noted in glomerular eluates and the role of the ribonucleoprotein anti- ribonucleoprotein system in sera is presently being studied. It has been observed that studies of antibody titers alone are inadequate for predicting the types of immune complexes deposited in the kidney. Either antigen excess could obscure detection of humoral antibody or extremely high titers of antibody as observed for ribonucleoprotein are not conducive to the formation of kidney localizing immune complexes. Immunofluorescent studies of complement deposits in kidney tissues indicate that variable amounts of Clg globulin are present, whereas C3 deposits are consistently found in nephritic kidneys.